Good engineering practices (API STD 520 and ASME B&PV
Code Section VIII) have long specified/required that inlet
piping pressure drop from the vessel to the safety relief
device should be limited to no greater than 3% of the safety
relief valves set pressure. Many companies have taken a
more lenient approach to the inlet pressure loss limits;
consequently, many installations do not meet the 3% design
guideline, as the prevailing company logic assumed that
existing installations were safe as long as the
inlet losses were less than the safety relief devices
blowdown with some built-in safety margin. Up until recent
fines by OSHA, there have been no hard and clear industry
requirements or penalties for companies to adhere to the 3%
inlet pressure loss rule. However, OSHA recently rejected
this argument and has now begun levying fines against
companies violating this 3% rule. In an April 2010 letter to
the API STD 520 Committee, OSHA stated that higher inlet
losses may be considered acceptable if safety relief valve
stability could be assured with an engineering analysis.

This monumental shift has added serious financial
consequences for violations of this rule, making compliance no
longer an academic argument. This article details a procedure
to assist facilities to ensure that existing
relief devices with inlet losses greater than 3% are properly
designed and will not chatter. It is not the goal of this
article to confirm the criteria for an installation to chatter,
but instead to give engineering guidance as to which
installations are acceptable as they are not expected to
chatter. To ensure that this methodology actually solves
problems associated with real installations, an entire refinery was subjected to the
methodology, and it was found that over half of the
installations that have inlet pressure losses greater than 3%
are acceptable as is and are not expected to chatter.

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